Scientists have cloned three calves that carry a foreign gene. The success, described in today's Science, opens the field for herds of transgenic cows that could produce copious amounts of milk rich in therapeutic proteins.
The standard way to create an animal with an extra gene is to inject the DNA into a fertilized egg. But most times the gene is not expressed and so the adult animal doesn't produce the desired protein. An alternate strategy is to first add the gene to the nucleus of a cell that development has specialized, then tuck that nucleus into an egg whose own nucleus has been removed. Last December, scientists succeeded in modifying sheep fetal cells this way to carry the gene for Human Factor IX, which some hemophiliacs take to aid blood clotting.
Now the same cloning technique has been applied to cows, which give much more milk. The researchers--James Robl, a developmental biologist and his colleagues at the University of Massachusetts at Amherst, and Steve Stice at Advanced Cell Technology in Worcester, Massachusetts--inserted a marker gene fused with a gene for resistance to the chemical neomycin into a culture of connective tissue cells called fibroblasts. Then they added neomycin to kill cells that hadn't taken up the gene combo. Those that survived developed into embryos, 28 of which were implanted into 11 cows; three gave birth to genetically identical calves, all of which carried the neomycin-marker gene.
Robl and Stice, in collaboration with the biotech company Genzyme of Cambridge, Massachusetts, have already created embryos that contain the human gene for albumin protein, which helps restore the blood's osmotic pressure after blood loss. "Albumin is at the top of everyone's list" of genes to clone into a cow," says Stice. A single cow could produce 80 kilograms of the protein a year, he says.
"This is very exciting study," says Randall Prather, a developmental biologist at the University of Missouri at Columbia. The neomycin strategy allows researchers to make sure that the gene is functioning properly before it's inserted into a cow, he says.